The sort of writers used in these emergency fill-in issues tend to be really, really, really, reeeeally bad.
Yes, that's really true. But in some cases, the opposite also happens. For example, Karl Kesel finished the work that Matt Fraction started in Fantastic Four. I didn´t read those stories, but Kesel is a great writer and great FF fan, so I think he did a good job. And when Robin Laws rescripted Mike Greel's latest Iron Man issues, he showed that it's possible to turn dirt into gold, only with dialogues.